Starlight can be surprisingly bright in the night sky. Of course, daytime is all about starlight. The star is our Sun and it shines so brightly because it is so close to us. Even the Moon is bright because of light from the Sun. Some stars produce much more energy than the Sun. They're just farther away so the light is not as obvious. How do stars generate so much energy?

Stars shine because of
nuclear fusion
. Fusion reactions in the Sun's core keep our nearest star burning. Stars are made mostly of hydrogen and helium. Both are very lightweight gases. A star contains so much hydrogen and helium that the weight of these gases is enormous. The pressure at the center of a star is great enough to heat the gases. This causes nuclear fusion reactions.

A nuclear fusion reaction is named that because the nuclei (center) of two atoms fuse (join) together. In stars like our Sun, two hydrogen atoms join together to create a helium atom. Nuclear fusion reactions need a lot of energy to get started. Once they begin, they produce even more energy. (
Figure
below
).

A thermonuclear bomb is an uncontrolled fusion reaction in which enormous amounts of energy are released.

Particle Accelerators

Scientists have built machines called
particle accelerators
. These amazing tools smash particles that are smaller than atoms into each other head-on. This creates new particles. Scientists use particle accelerators to learn about nuclear fusion in stars. They can also learn about how atoms came together in the first few minutes of the universe. Two well-known accelerators are SLAC, in California, and CERN, in Switzerland (
Figure
below
).

The SLAC National Accelerator Lab in California can propel particles a straight 2 mi (3.2 km).

Vocabulary

nuclear fusion
: When nuclei of two atoms fuse together, giving off tremendous amounts of energy.

particle accelerator
: A technologically advanced device that propels particles. This simulates conditions in the first minutes of the universe or in stars.

Summary

In a nuclear fusion reaction, two nuclei combine to form a larger nucleus.